I96 PORIFERA MTCROMASTICTORA chap. 



one kind or another to various Micromastictora, to retain them 

 together in a temporary class, the Myxospongiae. 



CLASS I. MYXOSPONGIAE 



The class Myxospongiae is a purely artificial one, containing 

 widely divergent forms, which possess a common negative char- 

 acter, namely, the absence of a skeleton. As a result of this 

 absence they are all encrusting in habit. 



One genus, Hexadella, has been regarded by its discoverer 

 Topsent l as an Hexactinellid. The same authority places 

 Oscar ella with the Tetractinellida ; it is more difficult to suggest 

 the direction in which we are to seek the relations of the 

 remaining type, Halisarca. 



Hexadella, from the coast of France, is a remarkable little rose- 

 coloured or bright yellow sponge, with large sac-like flagellated 

 chambers and a very lacunar ectosome. 



Oscar ella is a brightly coloured sponge, with a characteristic 

 velvety surface ; it is a British genus, but by no means confined 

 to our shores. Its canal system has been described by some 

 authors as diplodal, by others as eurypylous. Topsent 2 has 

 shown, and we can confirm his statement, that though the 

 chambers have usually the narrow afferent and efferent 

 ductules of a diplodal system, yet since each one may com- 

 municate with two or three canals, the canal system cannot 

 be described as diplodal. The hypophare attains a great 

 development, and in it the generative products mature. The 

 pinacocytes, like those of Plakinidae, and perhaps of Aplysilla, 

 are flagellated. 



Halisarca, also British, is easily distinguished from Oscarella 

 by the presence of a mucus-like secretion which oozes from it, 

 and. by the absence of the bright coloration characteristic of 

 Oscarella. It naturally suggests itself that the coloration in 

 the one case and the secretion in the other are protective, and 

 in this respect perform one of the functions of the skeleton of 

 other sponges. The chambers are long, tubular, and branched. 

 There is no hypophare. 



1 Mem. Soc. Zool. France, 1896, p. 119. 

 2 Arch. Zool. Exp. (3) iii. 1895, p. 561, pi. xxiii. 



